All:

After reviewing the iptables chains workflow, I discovered that the Nginx Worker (100.64.8.236:12345) outside interface was associated with the OUTPUT chain.

(192.168.0.2:12345) OUTPUT ==>
(192.168.0.154:$port) PREROUTING ==> (100.64.8.236:12345) POSTROUTING ==>
Windows Client (192.168.0.154:$port) ==> Nginx Master (192.168.0.2:443) | Nginx Worker (100.64.8.236:12345) ==> Upstream Desination Server (104.27.161.206:443)
<== POSTROUTING (192.168.0.2:443) <== PREROUTING (104.27.161.206:443)

Once adding the appropriate iptables OUTPUT rule, using the correct interface (vlan2), the packets leaving the Nginx Worker (100.64.8.236:12345) were then appropriately MARKed and routed to the OpenVPN Tunnel.

# iptables -t mangle -I OUTPUT -o vlan2 -p tcp -m multiport --sport 12345 -j MARK --set-mark 0x2000/0x2000
Now, I just need to figure out the Nginx SSL Client CA Trust configuration and we should be in business.

Hope this helps someone in the future.

Respectfully,

Gary